1. Field of the Invention
The present invention relates to a recording apparatus and, more particularly, to a serial type recording apparatus in which a stepping motor is used as a driving source for effecting at least the movement for the recording and scanning of a recording head.
2. Description of a Prior Art
Generally, in the serial type recording apparatus, a stepping motor or a brushless motor of the hybrid type or the PN (permanent magnet) type is frequently used as a motor to drive a carriage for conveying a recording head in order to record and scan.
For instance, in a brushless motor, for example, a Hall element is ordinarily used to detect the position of the magnetic pole of a rotor to control a current supply to the motor. An optical or magnetical type encoder is used to detect the speed of the rotor.
However, such a brushless motor has the following problems.
(1) It is necessary to match the positions of the stator magnetic pole and the Hall element.
(2) If the current supply is switched by the Hall element, since the positions of the Hall element and the stator are unconditionally determined, the current supplying method of the motor is fixed. For instance, in the cases where what is called a 180.degree. current supply control is executed and where what is called a 90.degree. current supply control is performed, the positions of the Hall element for the position of the magnetic pole of the stator electrically differ by 45.degree.. Therefore, in order to execute two kinds of current supply controls by using a single motor, the number of Hall elements must be doubled and the Hall elements must be arranged at positions suitable for the current supply controls, respectively.
For example, stepping motors in which the current supply control is controlled by using an output of an encoder have been proposed in JP-A-62-193548 and JP-A-62-193549. However, only a motor structure in which an encoder is arranged at a predetermined position is disclosed in the above citations and none of the drive control circuit and method of the motor and the like is disclosed.
Therefore, in U.S. Pat. No. 4,963,808, there has been proposed a control apparatus of a stepping motor, in which an encoder, having portions to be detected of the number which is an integer times as large as the number of magnetic poles of a rotor is fixed to the shaft of the rotor. The number of portions to be detected of the encoder in association with the rotation of the rotor is counted at a predetermined position on the stator side, and a current supply to a coil of the stator is switched when the count value coincides with a predetermined value. That is, the driving of the stepping motor is controlled by a closed loop.
Conventionally, the drive control for the stepping motor has been performed by an open loop control treating the number of driving pulses of the stepping motor and the frequency of such pulse.
In the case where such a conventional stepping motor which is driven by the open loop control is used as a carriage driving motor, when the carriage is driven and run, particularly, in the case of the hybrid type motor, an annoying noise like "kee---n" which is caused by the vibration of the rotor of the stepping motor is generated. On the other hand, when the carriage is started, stopped, and reversed, that is, when the stepping motor is started, stopped, and reversed, the stepping motor is started or stopped while vibrating, so that a large noise like "Gatan" is generated. The above noises cause a problem in a printer which hardly generates noises such as an ink jet printer, particularly, like a bubble jet printer or the like.
On the other hand, although the use of the above brushless motor as a carriage driving motor is also considered, in the case of the brushless motor, the rising time upon actuation is long and it is not suitable as a carriage driving motor in which the start, stop, reversal, and start of the motor are repeated for almost every line. In the case of using the brushless motor, high-speed recording cannot be performed.
Therefore, in U.S. Pat. No. 4,928,050, there has been proposed a recording apparatus in which a stepping motor is used as a driving source and the recording head is moved to record and scan, wherein the recording apparatus comprises: detecting means for detecting a rotational angle position of a rotor of the stepping motor; and control means for closed loop controlling the driving of the stepping motor in accordance with the result of the detection of the detecting means.
In the closed loop control of the stepping motor, an encoder is attached to the rotary shaft of the stepping motor, an output signal of the encoder is counted, the rotational position is detected, and a motor energization signal is switched when the count value coincides with a predetermined count value, thereby controlling the rotation of the stepping motor.
As mentioned above, in the case of driving the stepping motor by the closed loop control, it is necessary to execute the speed control and the position control in order to improve the recording accuracy. When the printer is designed, it is necessary to determine a control gain and a phase as parameters for the speed and position controls in consideration of the stability of the carriage speed, the response speed of the stepping operation, and the like.
In the recording apparatus in which the carriage is driven by the stepping motor which is controlled in a closed loop, if a load torque which is applied to the carriage motor increases due to an environmental change or a change due to aging of the system, the objective values of the rising time to actuate the carriage, driving speed, speed change amount, and the like cannot be satisfied as a result, the carriage stops in the worst case.
The load torque increases in the following cases. For instance, in a low temperature environment, the viscosity of lubricating oil for reducing the friction which coats the carriage sliding shaft deteriorates, or coefficients of thermal contraction are not matched due to a difference in the composition of the parts thereof, or paper particles, dust, and the like enter between the sliding shaft and the carriage, so that the friction load increases and the load torque of the motor is increased.
Actually, a load torque margin is provided so that the normal operation can be also executed even if the driving system changed. However, to provide a torque margin, it is necessary to use a motor which can output a larger generation torque or to reduce the load by using a reduction gear or the like. Thus, the number of parts increase, and the costs rise.